Extension cleanup: Move scripting whitelist to ExtensionsClient.

BUG=298586

Review URL: https://codereview.chromium.org/30533006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231188 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231188